1. Field of the Invention
The present invention relates to electronic devices and systems, and in particular, to oscillators, and devices/systems employing oscillators.
2. Description of Related Art
Many modern electronic devices/systems include oscillators, in particular voltage-controlled oscillators. There are many different types of voltage-controlled oscillators (VCOs), with one type being an inductor-capacitor (LC) oscillator. For very high frequency applications, LC VCOs with frequency stability and superior phase noise performance are desired.
The main components of a LC VCO typically include a differential LC resonant tank or LC tank which includes transistors. The amplitude of the oscillations of the LC tank is affected by Q of the LC tank and the tail current of the LC oscillator. The tail current or the current through the LC tank depends on the bias applied on the gate of the tail current device. Due to process variations, the current (or transconductance) through this tail current device may vary for a fixed bias voltage. Transconductance (gm), in field effect transistors, is the extent to which the drain current (ID) changes in response to a change in the gate-to-source voltage (VGS), which may be expressed as follows: gm=dlD/dVGS. Also, the quality factor Q may vary in the LC tank. The quality factor Q is defined as the reactance of the LC tank divided by resistance of the LC tank. If the Q of the LC tank degrades and/or the tail current decreases, the amplitude of oscillations also decreases. Reduction in the amplitude of oscillations will directly impact phase noise/jitter of the LC oscillator. Phase noise is instantaneous variations in the frequency of oscillation as caused by various loss elements, such as resistances which may vary with variations of temperature, load, and processing. Instantaneous variations in the frequency of oscillation may result in phase noise.